1. Field of the Invention
This invention relates to the manufacture of integrated circuits and more particularly to the doping of a layer of polycrystalline silicon.
2. Description of the Relevant Art
Polycrystalline silicon (also called polysilicon, poly-Si, or poly) deposited in thin film form has many important applications in the manufacture of integrated circuits. When doped with impurities, polysilicon exhibits low resistivity. Heavily doped polysilicon films are widely used as gate electrodes and interconnect lines in modern metal oxide semiconductor (MOS) transistors. Polysilicon is used in these applications for a number of reasons. Two important reasons are that polysilicon adheres well to silicon dioxide (oxide) insulating layers, and that polysilicon is able to withstand high temperatures involved in subsequent processing steps (e.g., source-drain implant anneal and thermal oxidation).
Dopant atoms may be introduced into a polysilicon layer during the formation of the polysilicon layer, by diffusion following formation, or by ion implantation. Polysilicon layers are typically not doped during formation due to the inability to closely control the thicknesses and doping uniformities of the layers. Diffusion doping involves the introduction of dopant atoms into an exposed surface of a polysilicon layer from a dopant-bearing ambient at elevated temperatures. During an ion implantation procedure, dopant atoms are ionized, accelerated to a velocity high enough to penetrate the surface of a silicon wafer, focused into a narrow beam, and scanned as a beam across an exposed surface of a polysilicon layer. Dopant ions impacting the surface enter the polysilicon layer and come to rest below the surface. A subsequent heating step is used to activate the dopant atoms within the polysilicon layer. Diffusion doping of a polysilicon layer is faster and cheaper than doping using ion implantation, and is therefore preferred. See, Wolf et al, Silicon Processing for the VLSI Era, Vol. 2, pp. 176-182 (incorporated herein by reference).
Phosphorus is a Group VA element and a common n-type dopant. A typical diffusion doping process used to introduce phosphorus atoms into a polysilicon layer is a batch process carried out in a diffusion furnace. Silicon wafers with polysilicon layers formed thereupon are placed on edge (i.e., vertically) in a wafer boat, and the wafer boat is placed within the diffusion furnace. A source of phosphorus atoms is introduced into the diffusion furnace and the temperature within the diffusion furnace is increased. Due to the closely-spaced arrangement of the silicon wafers in the wafer boat, all areas of the exposed surface are not equally accessible to phosphorus atoms. As a result, the uniformity of the doping of the polysilicon layer varies. It would thus be advantageous to have a polysilicon diffusion doping method which employs a highly uniform source of dopant atoms. Such a polysilicon diffusion doping method would result in highly uniform dopings of polysilicon layers.